"Since 2007, there have been four major airline mergers. As a result of this consolidation, about 85 percent of passengers in the U.S. flew on four domestic airlines in 2013. Certain industry observers have raised concerns that consolidation could have adverse effects on airline competition, such as higher airfares and reduced service. Others argue that consumers stand to benefit from recent changes in the industry as profitable airlines reinvest in new planes and expand their networks.To assist Congress in overseeing changes in the airline industry, GAO was asked to examine the state of competition in the domestic passenger airline industry. This report addresses (1) changes to the financial health of the U.S. airline industry since 2007; (2) changes to the structure of the market since 2007; (3) how consumers have been affected by these changes; and (4) views of stakeholders on the key challenges to airline competition and actions the federal government could take to address these challenges.GAO analyzed airline financial data reported to DOT, as well as DOT passenger itinerary data from 2007 through 2012, the latest year available. GAO interviewed DOT and DOJ officials and 26 stakeholders, selected based on prior work and their expertise in the field, from organizations in sectors such as academia, airlines, consumer advocacy, and finance"